1. Field of the Invention
The subject invention relates to spark plugs for igniting combustion gases in a combustion chamber of an internal combustion engine, and more particularly toward an extension type spark plug as used chiefly in stationary engine applications such as generators, pumps and the like.
2. Related Art
Extension type spark plugs are used in applications where the depth of the spark plug bore in the cylinder head requires the use of an unusually long spark plug. While specialty spark plugs can be designed for these applications, it is sometimes preferred to adapt extensions for conventional, long-life spark plugs for these purposes.
One approach to providing the necessary extensions has been to provide a kit for adapting conventional long-life spark plugs in the field by the addition of various combinations of shell extensions, upper terminal extensions and various combinations of elastomeric insulators to isolate the shell extension from the upper terminal extension. Skilled laborers must be employed to perform the required assembly due to the complexity of the operation. For example, attachment of the shell extension to the shell may use press-fit connections which are prone to coming apart in the field. Field assembly necessitates that any metallurgical bond used to attach the shell extension to the shell utilize relatively low temperature processes, such as soldering or low-temperature brazing which produce relatively weak mechanical joints which can later result in failure while installing or uninstalling the spark plug. So further, extension kits frequently utilize combinations of elastomeric and plastic insulating materials, as they are more durable with respect to field assembly and installation; however such materials do not necessarily provide desirable electrical isolation characteristics. Because field installed extensions are typically non-hermetic, moisture and other contaminants are accessible to the insulating materials, thereby diminishing their electrical isolation capabilities. Such materials also limit the maximum operating temperature of the extension spark plug based on inherent characteristics of materials such as a softening point or a glass transition point of the plastic materials used.
Very few factory-assembled extension spark plugs are known. Those that are known typically employ many of the same components and manufacturing methods described above, and thus are generally subject to the same limitations.
Therefore, it is desirable to manufacture extension spark plugs with improved reliability and an increased operating temperature range, as well as having enhanced thermal and electrical properties and resistance to environmental degradation.